High visible transmittance, low emissivity coatings upon transparent substrates such as glass are characterized by their ability to transmit visible light while minimizing the transmittance of other wavelengths of light, such as light in the infrared spectrum. This characteristic is particularly useful in applications where it is desirable to minimize radiative heat transfer without impairing visibility, such as in architectural glass or automobile windows. For asthetic reasons, in many such applications it is important to maintain reflectance relatively consistent throughout the visible spectrum so that the coating has a "neutral" color, i.e., is essentially colorless.
Generally speaking, such high transmittance, low emissivity coatings comprise a "film stack" having a thin metallic layer or film with high infrared reflectance and low transmissivity disposed between metal oxide layers. The metallic layer may be virtually any reflective metal, such as silver, copper or gold, with silver being frequently used. Metal oxides useful in high transmittance, low emissivity films include oxides of titanium, hafnium, zirconium, niobium, zinc, bismuth, indium and tin. Prior art systems have also employed oxides of metal alloys, such as zinc stannate or oxides of indium/tin alloys.
The metal oxide layers of such coatings serve two important functions. First, they serve to reduce the visible reflection of the film stack to enhance transmittance. The metal oxides used in these layers should therefore have a relatively high index of refraction, e.g., on the order of 2.0 or more, in order to achieve this end. Second, they should serve to protect the reflective metal layer from the environment. The film stack is frequently isolated from contact with the environment, as by disposing the film stack between two spaced panes of glass in a composite window structure. However, when these products are being assembled, the film stack is frequently subjected to relatively harsh conditions, such as by handling, shipping or washing of the panes prior to assembly.
A variety of attempts have been made to enhance the ability of the metal oxide layers to protect the reflective metal layer. For instance, Gillery et al. teach the use of titanium oxide as a protective overcoat in U.S. Pat. No. 4,786,563, the teachings of which are incorporated herein by reference. Gillery et al. explain that titanium oxides achieve the best results, but that such an overcoat could be formed of a metal rather than a metal oxide; titanium and alloys of iron or nickel are listed as prime candidates for such a metal layer. Gillery also teaches that certain other oxides simply lack the requisite durability to be used as such a protective overcoat. Zinc oxide, bismuth oxide and tin oxide are all listed as having undesirable properties, such as a lack of durability, which make them unsuitable for a protective overcoat.
However, it has been found that the use of a titanium oxide overcoat such as that taught by Gillery et al. is particularly prone to scratching or abrasion during shipping and washing operations. When coated substrates are washed before being assembled into a final product, the film stack comes into physical contact with a washing solution apparatus. It has been found that such a washing stage can physically abrade an overcoat of titanium oxide or the like, noticeably degrading the appearance of the finished article.
It would therefore be desirable to provide a film stack which provides suitable protection for the underlying metal layer, but can withstand the rigors of normal handling associated with such substrates.